Even Steve Earle admits that by the time you’re in your fifth decade of writing songs, inspiration can be tough to find.

Then came an invitation by Shawn Colvin, the Grammy-winning singer-songwriter. In late 2014 she asked Earle to join her for a week of shows together. Unlike most twin bills where two songwriters swap songs onstage, Colvin and Earle discovered that besides sharing a common folk root in their music, when they sang together it created a perfect blend.

“We sing cross harmony. We don’t rehearse it, we just naturally do it,” Earle says. “That’s what we weren’t prepared for. When you’re an artist our age” – he is 61, Colvin 60 – “you’re looking for surprises and they are harder and harder to come by.”

A few days later he proposed making a record. “I was like, ‘Oh sure’,” Colvin says. “You take that with a grain of salt because everybody’s busy. But if you know Steve Earle, he multitasks like crazy.”

Three songwriting sessions later and we have Colvin & Earle, a new album, just released, that both songwriters say represents not just a mutual collaboration but also a standalone musical project they hope will continue onward alongside their respective careers. A tour, featuring just the two of them, started this week in London and continues through late September throughout the US.

With distinguished careers of their own making, Colvin and Earle don’t necessarily need to share the bill with anyone. Earle is a troubadour in the mold of his early hero Townes Van Zandt; over the years, whether his music has veered from country to rock to bluegrass to folk, he has consistently written peerless melodies in songs that have spanned agitprop in the George W Bush years to literary-minded folk-rock. After a series of well-received folk-pop albums, Colvin won Grammys for both song and record of the year in 1997 for Sunny Came Home, and since then she has released music that evokes a blend of Joni Mitchell, her hero, and the Beatles.

On the new album, their contrast in vocal styles – his rough, hers tender – came together to create a single voice that neither expected to hear. Close melodic harmony singing has always served as a fundamental element of country music, from the Carter Family to the Louvin Brothers to the bluegrass of Bill Monroe. But unlike other famed male-female duet partners George Jones and Tammy Wynette or Porter Wagoner and Dolly Parton, Colvin and Earle’s singing is not a metaphor for the relationship between men and women. Instead, their closeness represents a kind of surrender to the other person in the room.

In conversation, both quote producer Buddy Miller, who plays on the album, as saying they each “take a step toward each other” when they sing, with Earle shifting into a quieter tone and Colvin rising in temper. “It’s not like I never sing softly, but I do change. And she’s reveling in the chance to get a little tougher. There are certain things that are expecting of her that she is rebelling against on this record,” he says.

Besides a cover of Ruby Tuesday by the Rolling Stones and Raise the Dead by Emmylou Harris, the songs are originals, written over time in Earle’s home in New York City, Colvin’s home in Austin, Texas, and a third session in Nashville, as well as ideas passed back and forth digitally via their phones. Collaboration did not come naturally, but they soon discovered that, once again, the spark they created in so short a time made the sessions effortless.

“Put me in a room with a songwriter and I just clam up,” Colvin says. “I get shy. My tendency is to take an idea and run off in a corner and Steve allowed me to do that. There wasn’t ego: we were opinionated, but it seemed to be in service of the same goal.”

The songs, like their creators, have maturity to them. The struggle of sustaining a lifelong partnership is a theme that runs throughout this album. “Can’t live without each other, can’t take it any more,” the pair sings on opener Come What May. “Something’s gotta give, but it never did before.” Opposed to sounding weary, the acoustic guitars and stomping beat suit a bright Beatles-esque melody. The simplicity is something Colvin said Earle pushed for in the writing.

“One of his mottos is ‘fear not the obvious’, which was revelatory to me,” she says. “Steve reminded me that if you’re agonizing over a lyric, let go of it for a minute and go in another direction.”

For the most part, Earle and Colvin sing as one, but for The Way That We Do, their vocals separate to describe a courtship told from two perspectives. There is reckless love, too – the upbeat pop number You Were On My Mind channels the anxiety of going through your day missing someone lost to time. On You’re Right, I’m Wrong, they pass regrets back and forth until the final line: “Maybe the truth is neither one of us ever loved anybody at all.”

Colvin says coming up with that line stunned them both. Then they laughed. After all, there are nine divorces between both songwriters, so they had little choice. “It was the pleasure of coming up with the line that was powerful,” she says.

Confronting trouble but not letting it lord over your life is another quality they share. Both are recovering addicts – Earle from heroin in the early 1990s and Colvin from alcohol, which she chronicled in a recent memoir. Both songwriters say their shared experience helped them communicate.

“We have a language we speak that comes from recovery and 12-step programs that not everybody speaks that makes everything easier,” Earle says. Colvin agrees: “When you’ve been through drug addiction and come out the other side, I just think there’s a sense of how you live and what you survived. And there is possibly a shorthand that comes with that. A compatibility that made it easier.”

Colvin & Earle appears amid what is sure to be a rollicking summer of political conventions and presidential debates – typically red meat for Earle, who was one of the most outspoken advocates against the Iraq invasion and subsequent pullback of civil liberties. The album has no such overtones, but Earle says he is an ardent supporter of Bernie Sanders and says he hopes he takes his message all the way to the Democratic convention. As for Republican nominee Donald Trump, he says, “it is dangerous to assume” he won’t get elected.

“Because he can. Reality TV has done a lot of damage to this country. I don’t watch reality TV. The last time I watched reality TV, I used to smoke crack and watch Cops. But when I stopped taking drugs, I stopped watching,” he says. “I don’t miss that.”

They will tour into the fall but then probably start up again next year. There is already talk of a second album. Earle says he feels comfortable in this new setting and it all goes back to what he heard onstage almost two years ago.

“I genuinely became a fan of those two voices and wanted to write songs for them,” he says. “We designed this as something we can the rest of our lives.”